1. Field of the Invention
This invention relates to an improved process for preparation of block copolymers from addition monomers incapable of anion initiated addition polymerization. More specifically, this invention involves a process for the heterophase sequential polymerization of addition monomers in preparation of block copolymers. This process lends itself to the preparation of block copolymers having predetermined electronic and physical properties.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on the imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging layer of said member in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light-struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on the image-bearing surface is rendered visible by development with a finely divided, colored marking material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface which have a polarity of charge opposite to the charge on the toner particles and thus form a visible powder image.
The developed image can then be read or permanently affixed to the photoconductor where the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder type photoconductive films (e.g. zinc oxide/insulating resin binder) where the photoconductive imaging layer is also an integral part of the finished copy, U.S. Pat. Nos. 3,121,006 and 3,121,007.
In so called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on the imaging surface of a reuseable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films and solvent or thermal fusion of the toner particles to a supportive substrate.
In the above plain paper copying systems, the materials used in the photoconductive insulating layer should be preferably capable of rapid switching from insulating to conductive to insulating state in order to permit cyclic use of the imaging surface. The failure of the material to return to its relatively insulating state prior to the succeeding charging/imaging sequence will result in an increase in the residual potential of the photoconductor. This phenomenon, commonly referred to in the art as "fatigue" has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling imaging system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of superior photosensitivity.
In addition to anthracene, other organic photoconductive materials, most notably, poly(N-vinylcarbazole) have been in the focus of increasing interest in electrophotography, U.S. Pat. No. 3,037,861. Until recently, neither of these organic materials have received serious consideration as an alternative to such inorganic photoconductors as selenium, due to fabrication difficulties and/or to a relative lack of speed and photosensitivity. The recent discovery that high loadings of 2,4,7-trinitro-9-fluorenone in poly(vinylcarbazoles) dramatically improves the photoresponsiveness of these polymers has led to a resurgence in interest in organic photoconductive materials, U.S. Pat. No. 3,484,237. Unfortunately, films prepared from poly(vinylcarbazoles) have poor mechanical properties (e.g. brittleness and relatively inflexibility). The addition of high loadings of activators, such as those described in the '237 patent, further impair the mechanical properties of such films.
The orientation of vinylcarbazole polymers is generally known to improve the mechanical properties of these materials, U.S. Pat. No. 2,215,573. However, where a film of poly(N-vinylcarbazole) is mechanically uniaxially oriented, the charge transport properties of the resultant film are somewhat impaired. During such uniaxial orientation, pendant carbazyl functional groups are believed to be spatially constrained within the composition. Such modification in the steric relationship of adjacent pendant carbazyl groups is also believed to be responsible for the deterioration in charge transport properties of the polymer.
One method commonly employed for enhancement of mechanical properties of a polymeric material is to form a block copolymer of said polymeric material with an elastomer. One of the more efficient of the reported methods for preparation of such block copolymers involves the addition of a monomer to a "living" polymer (prepared by unterminated anionic polymerization techniques); see F. W. Billmeyer, Textbook of Polymer Science, 2nd Edition, Chapter 10 and 11, Wiley-Interscience Publishers, (1971) and associated bibliography. The addition of an addition monomer to a living polymer leads to the formation of di-block copolymers substantially free from contamination by homopolymer fractions. The use of a di-functional living polymer will lead to the formation of triblock copolymers. Unfortunately, N-vinylcarbazole and its analogues cannot be polymerized anionically. Thus, they cannot be formed into living polymers nor can they be copolymerized with living polymers since the polymerization mechanism is substantially the same in both instances.
Accordingly, it is the object of this invention to remedy the above as well as related deficiencies in the prior art.
More specifically, it is the object of this invention to provide a process for synthesis of photoconductive polymers having enhanced mechanical properties.
It is the principal object of this invention to provide a process for preparation of block copolymers from monomers generally regarded as incapable of anion initiated polymerization.
It is another object of this invention to provide a process for preparation of block copolymers containing photoconductive segments wherein said copolymers are substantially free of impurities thus eliminating or minimizing the separate extraction of such impurities from the copolymer.
It is yet another object of this invention to provide a process for enhancing the flexibility of photoconductive polymers by forming block copolymers of such materials with elastomeric materials.
It is still yet another object of this invention to provide a process for reducing the brittleness of photoconductive polymers by forming block copolymers of such materials with elastomeric materials.